As the primary organ of vision in vertebrate animals, the eye often compared to the usual photographic camera. In simplified terms, the eye has a lens system, a variable aperture system (the pupil) and a light-sensitive retina that corresponds to the film. The focusing ability of the eye resides within the crystalline lens which comprises a strong elastic capsule filled with viscous, transparent fibers of crystalline and albuminoid proteins. Far more complex and fragile than that of a mere camera lens, the crystalline lens of the eye is especially prone to damage due to disease, environmental factors and the aging process. For a review on the lens system, see, for example, Cotlier, E. in "Adler's Physiology of the Eye: Clinical Application", 8th edition, R. A. Moses and W. M. Hart, editors; C. V. Mosby Co.; St. Louis, Mo., USA (1987), pages 268-272.
The unique ability of the lens to change its shape or curvature to automatically adjust the focus of the eye for objects at different distances is known as accommodation. Accommodation occurs by muscular contraction and relaxation causing the elastic-like lens to change shape and increase its optical or refractive power. For a general discussion of the mechanism of accommodation, see Guyton, R. C. "Medical Physiology", 6th edition, W. B. Saunders Co.; Philadelphia, Pa., USA; 1981; page 724-735; Toats, F. M. Physiol. Rev. 52: 828, 1972; and Witkovsky, P., Annu. Rev. Physiol. 33: 257, 1971.
Accommodation is lost during the aging process primarily due to a hardening of the natural lens. Loss of accommodation due to a progressive denaturation of the lens proteins produces an abnormal condition commonly known as "presbyopia". Presbyopia generally affects individuals in the early to mid forties and the resultant gradual loss of visual acuity is generally treated with bifocal spectacles. For general discussion of presbyopia, see "Merck Manual" 15th Edition, Merck, Sharpe and Dohme publishers, 1987, page 2211 and Guyton, R. "Medical Physiology", 6th edition, W. B. Saunders Co.; Philadelphia, Pa., USA; 1981; page 724-735.
Lenticular cataracts are an especially common eye abnormality characterized by a progressive loss of vision beginning in middle age or later. In the early stage of cataract formation, the transparent protein fibers of the lens become denatured, presumably by oxidative damage due to the normal aging process. The denatured proteins then coagulate, forming the characteristic opaque areas in place of the normal transparent protein fibers of the lens. In the advanced stage, calcium deposition occurs on the coagulated proteins, thus further increasing the opacity. Cataract formation can also be accelerated by exposure to X-rays, heat from infrared rays, systemic disease (e.g. diabetes), uveitis, or systemic medications (e.g. corticosteroids). The degree of vision loss due to cataract formation can be ascertained by ophthalmoscopic slit-lamp examination which provides more details about the character, location, and extent the opacities. For a detailed discussion of cataract formation, see Van Heynengen, R. Sci. Am. 233(6): 70, 1975 and "Merck Manual", 15th Edition, Merck, Sharp and Dohme publisher, 1987, page 2227.
Frequent refraction corrections help maintain useful vision during cataract development. When a cataract has obscured light transmission so greatly that useful vision is lost, surgical intervention is necessary to extract the lens. Lens extraction can be accomplished by total removal of the lens, or by phacoemulsification of the lens followed by irrigation and aspiration, leaving the lens capsular sac intact. When this is done, however, the eye loses a large portion of its refractive power. Post-operative refractive correction to alleviate the visual defect is accomplished by cataract spectacles, contact lens, or intraoperative implantation of an intraocular lens.
Traditionally, cataract spectacles have produced less than satisfactory results because of induced visual distortions such as aberrant depth perception. For example, cataract spectacles with thick lenses are known to induce a Galilean telescopic effect which results in abnormally magnified images. Moreover, unilateral surgical removal of lenses makes correction of stereovision by such spectacles virtually impossible.
Contact lenses eliminates many of the aforementioned problems, however, the magnification problem remains. Furthermore, many patients are unable to tolerate contact lenses because of poor manual dexterity, insufficient tear production or lens hygiene problems.
Implantation of endocapsular lens or (as more commonly known) intraocular lens (IOL), on the other hand, has been widely accepted as the treatment of choice for correcting visual impairments following removal of diseased lenses. The remarkable success of IOL implantation is due to significant improvements in surgical instrumentation and technique as well as in the design and construction materials of IOLs.
Microsurgical procedures have been developed to remove cataract lenses through very small incisions in the capsular sac (see, for example, Arshinoff, S. A. Curr. Can. Ophthalmic Pract. 4(2):64, 1986 and Welsh, R. C. et al. Cataract Surg NOW 1(2): 21-22, 1983 for a discussion of capsulotomy surgical techniques). An IOL is then gently inserted into the intact capsular sac, positioned in place, and the wound is then closed with fine sutures. Conventional IOLs are generally fitted with surgical loops to fix and/or maintain the IOL in position. Materials which are used to fabricate the IOLs are typically rigid or semi-rigid plastics such as polymethyl methacrylate. Newer and softer fabrication materials include biocompatible hydrogels and silicones. For a general discussion of IOL development, see for example, Apple, D. J., Geiser, S. C. and Isenberg, R. A. "Evolution of Intraocular lenses," University of Utah Printing Service, 1985).
Conventional IOLs, however, have a number of deficiencies associated with their use. For a review of the complications of conventional IOLs, see for example, Apple, D. J. et al., (1984) Ophthalmology, Vol. 29, No. 1; Drews, R. C. (1982). Trans. Ophthal. Soc. U.K., Vol. 102, page 498; DeVore, D. P. (1991) J. Long-Term Effects of Medical Implants, Vol. 2, in press. For example, implantation of conventional IOLs are known to induce excessive accumulation of epithelial cells lining the lens capsule. This interface of cells, in turn, results in opacification of the lens as well as a variety of pathological conditions which include pupillary occlusions, iris atrophy and secondary glaucoma. Moreover, Mechanical dislocation of the IOL frequently results in damage to the corneal endothelium. Another notable drawback is that none of the IOLs of the prior art have accommodative capacity.
With the capsular bag intact, a safe, effective injectable material that could be used to refill the capsular bag and which simulates the natural lens would be desirable. This material should have an index of refraction similar to that of the natural lens, but variable so that any refractive errors might be corrected. Such injectable lens would still be able to accommodate and therefore provide a dramatic advantage over current intraocular lens implants.